This invention relates to a laser of the kind in which a capillary bore confines the discharge within the cavity.
This invention relates particularly to a spider structure for preventing transverse movement of the free end of the bore with respect to the outer envelope or jacket while permitting longitudinal movement resulting from the differential thermal expansion of the bore with respect to the envelope which occurs during laser operation.
Gas lasers, such as helium-neon gas lasers, use a capillary bore for confining the discharge to maximize the power output.
In the coaxial design of such lasers an outer envelope or jacket surrounds the cavity, the anode and cathode are located at opposite the ends of the outer envelope, end mirrors are mounted on the ends of the envelope, and the capillary bore tube is mounted within the outer envelope and in alignment with the end mirrors. One end of the capillary bore tube is connected to the end of the outer envelope so as to be fixed in position. The other end of the bore is called the free end and projects within the cavity toward the end of the envelope opposite that to which fixed end of the bore is attached.
The bore tube is heated (by the operation of the laser) to a substantially higher temperature than the temperature reached by the outer envelope. Since the bore tube is subjected to a higher temperature than the outer envelope (as the laser is energized from a non-operating condition to an operating condition), the length of the bore tube increases at a greater rate than the length of the outer envelope. The free end of the bore tube must therefore be permitted to shift longitudinally with respect to the outer envelope to accommodate this difference in thermal expansion.
The laser light in this kind of laser has a Gaussian profile. The profile is symmetric about the central axis. While the Gaussian profile distributes far away from the central axis, the profile drops off rather sharply with distance from the central axis for the laser modes produced in tuned lasers of this kind.
The nominal design with lasers of this kind is to have the internal diameter of the bore in a range of three to four times larger than the laser mode size w, which is the 3 dB point on the Gaussian intensity profile (so that the bore will produce a maximum enhancement of the power output while still providing enough leeway to accommodate tolerances that must be accepted in fabricating laser tubes of this kind). It is not practical to make the bore size much larger than this three to four times mode size range, because the capability of the laser is not fully utilized with such larger bores.
Using bores in this size range does, however, make it quite critical that the free end of the bore be maintained in proper radial alignment within the laser, because if the free end of the bore shifts transversely by even minute amounts, say 1/2 mil (0.001 inch) with conventional, existing helium neon lasers, that little amount of transverse or radial shifting is enough to change the power by five to ten percent, or more, depending upon the design of the laser.
The loss in power results from a chopping off of part of the Gaussian distribution of the light when the free end of the bore shifts transversely within the cavity.
The longer the tube is, the more prominent the problem is.
Transverse shifting of the free end of the bore can, in many instances, cause a permanent change in power output. For example, if the laser is dropped of otherwise jarred, the shock load can cause a substantial and permanent change in the power output if the free end of the bore is not adequately supported against transverse movement.
In the prior art a flexible, spring-type spider construction has been used to support the free end of the bore. The flexible, spring-type spider used in the prior art extended between and engaged the inside surface of the outer envelope and the outside surface of the bore to position the free end transversely within the cavity. The flexible, spring-type spider was not rigidly attached to the envelope and the bore, but instead permitted the free end of the bore to slide within the flexible, spring-type spider for accommodating the required longitudinal movement of the free end resulting from the differential thermal expansion as noted above.
The problem with the flexible, spring-type spider support of the prior art was that it did not provide a sufficiently rigid support in the transverse direction. Relatively high shock loads and/or relatively high vibration could (as a result of cocking of the spring-type suspension or for other reasons) cause a permanent change in the transverse position of the free end within the cavity such that the power output of the laser would be reduced by an unacceptable amount.